The invention concerns a telecommunications method and system, in particular ones using packet mode, in which it is necessary to transfer calls simultaneously from one path to another.
The transfer of calls from one path to another is sometimes referred to as xe2x80x9chandoverxe2x80x9d or xe2x80x9chand-offxe2x80x9d.
In some communications systems it is necessary for signals to be handed over from one path to another during a call or connection.
A first example of this is a mobile telephone system currently under development, usually referred to as the Universal Mobile Telephone System (UMTS) and in which each mobile station is connected to the network by a fixed base transceiver station radiating in a particular geographical area. The call must be handled by another base transceiver station when the mobile station moves away from that area. The two base transceiver stations are connected on their upstream side to the same switch. Accordingly, in this case, a call between a terminal and a switch must be handed over from a first path via a first base transceiver station to a second path via a second base transceiver station.
A second example is a telecommunications system using a constellation of non-geostationary satellites in low or medium orbit. The orbits are chosen so that practically all of the surface of the Earth is covered, in other words so that at least one satellite can be seen at any time from any point on the Earth (sometimes with the exception of polar areas). Because the satellites are not geostationary satellites, each point on the Earth sees the same satellite for only a limited time, in the order of 15 minutes at most. The telecommunications system is therefore organized so that, from the point of view of a terrestrial user, when one satellite leaves the area of visibility there is another satellite ready to take over the call. In each area, having a diameter of several hundred kilometers, for example, a connection station connects each terminal to the network, and the station and the terminal communicate via a satellite. In this case, the call between a terminal and the connection station is effected initially via a first satellite (first path) and subsequently via a second satellite (second path).
In all such systems, information is transmitted in digital form in cells or packets. The cells comprise a particular number of bits, for example, as in the ATM (Asynchronous Transfer Mode) standard. The packet length can vary.
Because the transmission times are generally different on the two paths, when a call between two particular points, for example a terminal and a connection station, is handed over from one path to another, a cell or packet on the second path can arrive before an earlier cell or packet on the first path, and it is necessary to retransmit the packets or cells in the correct order. Also, handover must not cause any cells or packets to be lost.
In circuit mode, the problem of the order of the cells or packets is solved by dating the transport frame or transmission medium. In other words, each cell or packet corresponds to a given (dated) section of the transmission medium.
This dating solution entails transmitting additional data. Also, it cannot be applied in packet mode because, in this case, the data can belong to different streams of information which are asynchronous and may not correspond to dates of the transmission medium. For example, the same transmission may convey information of different kinds with different bit rates and possibly different priorities.
To solve the problem of the order of the cells or packets at the receiver, the last cell on the first path can be transmitted with a mark, and two receivers can be provided, one for each path, each having a demodulator and a buffer. The buffer of the second path receiver delays the cells received until the last cell of the first path, which is identified by a mark, has been received and processed. Once the last cell has been received, the buffer of the second path receiver is released.
That prior art technique has the drawback of necessitating a receiver with two demodulators and two buffers, one for each path. That disadvantage is particularly serious in the case of a terminal for consumer applications, which must be of low cost. Also, marking the last cell on the first path constitutes signaling that mobilizes limited communications resources.
The invention addresses those drawbacks.
In the transmitter cells or packets intended for the second path are transmitted only after the last cell or packet intended for the first path has been transmitted, and in the transmitter and/or in the receiver cells or packets on the first or second path are delayed so that cells or packets on the second path arrive after cells or packets on the first path.
It is therefore unnecessary to transmit signaling for the last cell on the first path.
Also, the cells or packets on the second path arrive after those on the first path, and it is therefore unnecessary to provide two demodulators. A single demodulator is sufficient.
In an embodiment of the invention, the transmission times on the first and second paths are made equal. To this end, buffers in the transmitter and/or the receiver delay the cells or packets for long enough to obtain equal transmission times, for example.
The buffers are the same as those used to construct queues, for example. They can equally well be separate from those queue memories.
The invention concerns not only a telecommunications method and system but also a transmitter and a receiver for implementing the method.
The transmitter includes two buffers, one for each path, and it includes means for preventing transmission of data at the output of the second buffer and said means are inhibited after transmission of the last cell or packet on the first path.
In one embodiment of the invention, transmitting the last cell or packet on the first path unblocks transmission of cells or packets from the second buffer. Alternatively, the transmission of cells or packets from the second buffer is blocked for a particular time period following the handover command sufficient for the first buffer allocated to the first path to be emptied by transmitting all the cells or packets on the first path.
A receiver in accordance with the invention includes only one demodulator.
The method, the transmitter, and the receiver of the invention can be applied equally well to handing over a single connection or a plurality of connections. However, in the latter case (handing over a plurality of connections), the invention has additional particular features that address the drawbacks of prior art solutions.
Prior art solutions to the problem of packet mode handover from one path to another also have the disadvantage that they take some considerable time to complete handover if there is a multiplicity of connections. This is because:
The cells or packets to be transmitted are held in buffers and there is a buffer for each particular type of waiting data. For example, one buffer (queue) is provided for cells or packets corresponding to telephone conversations, another for electronic mail, and another for image data.
The various queues generally have different rates or speeds. Because the command to hand over or transfer cells or packets is executed on the upstream side of the buffers, the time to transmit the cells or packets to the output of the buffers varies from one buffer to another, i.e. from one connection to another. The time at which each cell or packet is transmitted cannot be predicted, because it depends on the content of each of the buffers and the grade of service each of them is allocated.
Given the above conditions, the time period during which the data is handed over from one path to another cannot be minimized, and this lack of synchronization complicates the transmission of the data.
The invention therefore also provides a method of transmitting non-dated or non-numbered digital data in packet mode between a remote transmitter and a remote receiver in which the data can take at least two different paths between the transmitter and the receiver and handover from a first path to a second path occurs during transmission, a set of buffers temporarily stores the data to be transmitted, and each buffer corresponds to a particular type of data to be transmitted. In the method, handover from the first path to the second path is practically synchronous for the various buffers of the system, and the data on the first and/or the second path is delayed in the transmitter and/or in the receiver so that the data intended for the second path reaches the receiver after the data transmitted on the first path.
The fact that handover is synchronous for the various buffers reduces the overall transfer time to zero.
Moreover, as already indicated, because reception via the first and second paths is separated in time, the receiver can include a single demodulator.
As in the case of a single connection, it is not necessary to transmit signaling for the last cell on the first path.
In one embodiment of the invention there is a single set of buffers and handover from one path to another occurs after the output of the buffers but before the multiplexing of the cells or packets leaving the various buffers. In other words, there are two multiplexing systems, one for each path. In this case, all the outputs of the buffers, i.e. all the connections, are handed over at the same time.
Because handover of data from one path to another is commanded from the output of the buffers, the time of handover is clearly defined and can be independent of the transit times of the cells or packets in the buffers.
The handover mechanism is simple to implement because the multiplexing systems are independent of each other and there are therefore no constraints on allocating powers and communications resources.
In a different embodiment of the invention, there is a set of buffers for each path, and during a duplication period before handover the two buffers receive the same cells or packets, handover from one path to another occurring either when the contents of the two buffers are identical or a predetermined time after the start of duplication.
The solution whereby handover is commanded a particular time after the start of duplication has the advantage that the handover time is clearly defined and so handover is easy to synchronize for all the buffers corresponding to various grades of service.
This embodiment can be used in particular when the handover time is known in advance, as is the case in a telecommunications system using satellites in low or medium orbit.
Note that, if handover from one path to another occurs when the contents of the two buffers are identical, in order to synchronize all the buffers, it is necessary to wait for the contents of all the pairs of buffers to be identical.
In a different embodiment of the invention, in which there is also a set of buffers for each path, cells or packets are transferred from each buffer of the first set (corresponding to the first path) to the corresponding buffer of the second set just before handover from the first path to the second path. However, because it necessitates the transmission of a large quantity of data in a short time period, this embodiment of the invention is more complex than the others. In contrast, the other embodiments do not necessitate any transfer of data from one buffer to another. In some cases, it may be beneficial to transfer signaling, in particular to detect when the contents of the buffers are identical, but such detection does not necessitate any transfer of data as such, only transfer of signaling, for example cell or packet numbers.
The invention therefore provides a method of transmitting non-dated or non-numbered digital data in packet mode between a remote transmitter and a remote receiver in which the data can take at least two different paths between the transmitter and the receiver and handover from a first path to a second path occurs during transmission. In the transmitter cells or packets intended for the second path are transmitted only after the last cell or packet intended for the first path has been transmitted and in the transmitter and/or in the receiver cells or packets on the first or second path are delayed so that cells or packets on the second path reach the receiver after cells or packets on the first path.
In an embodiment of the invention, a first buffer temporarily stores data to be transmitted on the first path, a second buffer temporarily stores data to be transmitted on the second path and transmission of data from the second buffer is blocked until the last cell or packet has been transmitted on the first path.
For this purpose, the last cell or packet on the first path can be marked in the transmitter, in which case the mark is used to unblock transmission on the second path and is eliminated for transmission to the receiver. Alternatively, transmission on the second path is blocked for a predetermined time period from the start of the command to hand over from the first path to the second path.
In one embodiment of the invention, to enable reception of cells or packets on the second path after reception of cells or packets on the first path, the cells or packets on the first or second path are delayed long enough for the transmission times on the two paths to be equal.
The transmission times can be made equal using transmit and/or receive equalization buffers.
In one embodiment of the invention, a set of buffers temporarily stores data to be transmitted, each buffer corresponds to a particular type of data to be transmitted, for example a particular grade of service and/or a particular flow, and handover from the first path to the second path occurs in a practically synchronous manner for the various buffers of the set.
For this purpose, the cells or packets can be delayed long enough for the transmission times on the first and second paths to be the same.
The buffers for temporarily storing data to be transmitted and/or received can be used to delay the cells.
In one embodiment of the invention, handover occurs at the output of the buffers before multiplexing the cells or packets for transmission. The two paths can be multiplexed independently of each other for this purpose.
In another embodiment of the invention, there are two sets of buffers, one for each path. To use this facility before handover from the first path to the second path, the two sets of buffers can be filled with the same cells or packets, handover occurring after the contents of the buffers are identical. In this case, handover can be effected a predetermined time from the start of filling the buffers with the same cells or packets.
Handover can also occur when it is established that the contents of the buffers are identical. This is established when the first duplicated cell or packet of the last of the connections has been transmitted on the first path, for example.
When the two sets of buffers are filled with the same cells or packets before handover, the cells or packets already transmitted on the first path are preferably eliminated from the second set of buffers between transmission of the last cell or packet on the first path and the start of transmission on the second path.
The invention also concerns a receiver adapted to receive digital data transmitted by the above method and includes only one demodulator.
The invention also provides a transmitter for implementing the above method which includes two buffers, the first of which is for temporarily storing non-dated or non-numbered cells or packets intended to be transmitted on a first path and the second of which is for temporarily storing non-dated or non-numbered cells or packets intended to be transmitted on a second path, and means for preventing transmission of data at the output of the second buffer which are inhibited after transmission of the last cell or packet on the first path.
In an embodiment of the invention, the transmitter includes means for marking the last cell on the first path with a mark which is used to command inhibiting of the means preventing transmission of data on the second path. In this case, it is preferable if the mark is eliminated before transmitting the last cell or packet on the first path.
In another embodiment of the invention, the means for preventing transmission of data at the output of the second buffer comprise time-delay means such that the means preventing transmission of data from the second buffer remain active for a particular time period after the command to hand over from the first path to the second path.
In an embodiment of the invention, at least one equalization buffer delays cells or packets on the first or second path long enough for the transmission times on the two paths to be equal. To this end, the temporary storage buffers can also be used for equalization.
The invention also provides an application of the method to a telecommunications system using a constellation of satellites in low or medium orbit in which all the terminals communicate with a connection station via a satellite and another satellite takes over the call, in particular when the first satellite begins to leave the area of visibility of the terminal(s) concerned.
The invention also provides an application of the method to a cellular telecommunications system in which each cellular area includes a base transceiver station for connecting the terminals to the network via a switch, the terminals communicate via a base transceiver station and a call is handed over from a first path via a first base transceiver station to a second path via a second base transceiver station.